Mast cells and eosinophils are closely associated in allergic reactions. Eosinophil peroxidase (EPO) when combined with hydrogen peroxide (H2O2) and a halide forms a strong oxidative system capable of initiating mast cell degranulation. EPO also binds firmly to the secreted mast cell granules (MCG) to form a complex with augmented peroxidatic activity. Objectives of this research include: 1) to study the role of the EPO-H2O2-halide system and oxygen metabolites (superoxide, H2O2, hydroxyl radicals, singlet oxygen) generated by intact eosinophils or by the model acetaldehyde-xanthine oxidase system in the induction of secretion by rat peritoneal mast cells. The effect on mast cell structure by these systems will be studied by transmission, freeze fracture and scanning electron microscopy; 2) to study the formation of MCG/EPO complexes by appropriate stimulation of intact eosinophils and mast cells; 3) to study the biologic properties of the MCG/EPO complex compared to free EPO and MCG in its ability in vitro to a) inactivate slow reacting substance of anaphylaxis and the chemotactic factors C5a and formylmethionyl-leucyl-phenylalanine, b) degrade heparin, c) kill a line of mouse ascites tumor cells (LSTRA cells) and d) kill the schistosomula of Schistosoma mansoni; the target cells will also be examined by scanning and transmission EM for evidence of cytotoxicity by the mast cell, MCG and MCG/EPO systems.